Electrical components in micromechanical devices normally include regions that are sensitive with respect to aggressive media. Thus, for example, contact connections such as bonding wires or electrical lines as well as arc gaps or circuits may corrode over time if they are attacked by aggressive media. In electrical components equipped with sensor elements, moreover, the signal acquisition may be impaired if deposits interfere with signal detection. Thus it is conceivable that such deposits alter the stiffness of the diaphragm used in a pressure sensor, as a result of which there is no longer a clear correlation between the pressure and a detected deflection of the diaphragm.
For protection against damaging environmental influences, it is possible to cover the electrical component, the electrical contacts and the sensor element at risk with a special passivating layer. This is done in such a way for example that the sensor element or the (electrical and/or mechanical) components required for detecting and/or evaluating a sensor signal are mounted in a housing and are subsequently covered by a passivator. Usually this passivation is achieved by filling the housing. The filling in this context is used to passivate the sensor element or to protect the components against media such as water, air, gasoline, salt etc. Thus it is possible to prevent sensitive elements of the sensor from corroding. What is problematic about the passivation, however, is the interaction of the passivator and the damaging medium.
Micromechanical pressure sensors, in which for system-related reasons the pressure is supplied from the front side of the sensor chip, are normally protected from environmental influences by a gel such as, for example, a fluorine silicon gel. This gel covers the surface of the chip and the bonding wires and prevents corrosive media from coming into contact with the chip. When selecting the gel, however, one must be mindful of the fact that the gel transmits the pressure of the medium for detecting a pressure variable to the pressure sensor diaphragm in the sensor chip.
For the application of pressure sensors in a highly corrosive environment, as can normally be found in the exhaust branch of a motor vehicle, even the best of the currently available gels cannot prevent corrosive ingredients of the medium from diffusing through the gel over time and resulting in a corrosion of the sensor element or of other components on the sensor chip.
An expensive structural variant for protecting the pressure sensor is to install the sensor element made up of a sensor chip and bonding wires in a chamber filled with silicon oil, which maintains contact with the environment via a steel diaphragm. A change of the ambient pressure is transmitted via the steel diaphragm directly to the silicon oil and thus to the sensor element or the sensor chip.
Non-prepublished German Application No. 102004006212 describes the use of electrical components in corrosive environments by introducing an additional layer of material into the passivator. This additional material layer reduces or completely compensates the diffusion of the corrosive components of the medium.